1. Field of the Invention
The novel nozzle which is the subject of the present invention broadly concerns the decarburization of molten pig iron by means of lances which are disposed above the level of the molten pig iron and which emit a jet of oxygen through a nozzle towards the surface of the molten pig iron. The invention more particularly concerns the characteristics of the jets of oxygen which issue from injection lances provided with the novel nozzle and the influence of those characteristics on the conditions in respect of interaction between the oxygen jets and the liquid pig iron. While not limited thereto, the novel nozzle is used in particular for the decarburization of chromium containing pig iron.
2. Description of the Prior Art
A number of studies have been carried out in regard to the characteristics of nozzles which are mounted at the end of oxygen injection lances used for the decarburization of pig iron. Thus, in the work which concerns the metallurgy of steel, using basic converters, entitled "BOF Steelmaking", published in the United States in 1976 by the Iron and Steel Society, precise information is set out in Volume 3, chapter 8, pages 150 to 153, concerning the characteristics of the nozzles mounted on oxygen lances. It is stated in particular at lines 28 to 35 of page 150 that a reasonably uniform supersonic flow can be achieved by means of a simple divergent conical portion following the constriction. FIG. 8-5, on page 151 reproduces such an oxygen lance nozzle. Finally, lines 7 to 17 on page 153 set out precise information concerning the half-angle which should be adopted, at the apex of the divergent frustoconical portion. Excessively large angles must be avoided, as they reinforce the shock waves causing excessively rapid dispersion of the jet. It is proposed that the half-angle at the apex of the injection nozzle be selected to lie within the range of between 2.5.degree. and 10.degree., a half-angle of 5.degree. being considered a practical compromise.
Experience has shown that nozzles constructed in that manner give relatively satisfactory results in the general case of decarburization of ordinary pig iron but, in contrast, difficulties have been met when treating chromium containing pig iron and in particular when carrying out the decarburization process which is the subject of French patent application No. 80 01809 filed on 24.01.80 by UGINE ACIERS, and corresponding U.S. application Ser. No. 06/221,903, filed Dec. 31, 1980 in the name of Georges Marizy, now U.S. Pat. No. 4,324,584, Apr. 13, 1982, the disclosure of which is hereby incorporated by reference.
That application describes and claims a process for the decarburization of chromium or nickel-chromium pig iron containing from 1.5 to 8% by weight of C, from 10 to 30% by weight of Cr and up to 30% by weight of Ni, which includes, at least in the final phase of the decarburization operation, the formation of a gas/molten pig iron emulsion within which carbon is directly oxidized by oxygen.
The conditions required for producing the abovementioned emulsion are critical. Indeed, it is known that producing an emulsion as between a molten pig iron, a slag and a gaseous phase is a relatively simple matter. This is the case with decarburization of molten pig iron by the Linz-Donowitz (LD) process. It is the viscosity characteristics of the slag which play the most important part, as is demonstrated in the article by A. CHATTERJEE, N. O. LINDFOR and J. A. WESTER: "Process Metallurgy of LD Steelmaking", Iron Making and Steel Making 1976, No. 1. In contrast, in order that an emulsion between a gaseous phase and the molten pig iron can be formed and maintained in a stable manner, even in the virtually total absence of slag, until the carbon content is reduced down to a final level of close to 0.2%, it is necessary to have clearly defined conditions in respect of temperature of the molten pig iron, carbon content, lance-bath distance, and oxygen flow rate and pressure.
As described in French patent application No. 80 01809, when the favorable conditions are combined and the emulsion as between the gaseous phase and the molten pig iron is formed and satisfactorily maintained, the result is both rapid and extensive decarburization and at the same time a particularly high chromium metal yield. The examples given in the abovementioned application concern tests carried out on unitary quantities of chromium containing pig iron of about 60 kg. The nozzle used had a neck diameter of 2 mm and an oxygen flow rate of 168 Nl/min.
The studies undertaken and the many tests carried out for developing that chromium containing pig iron decarburization process have shown that it was not sufficient to suitably adjust the operating parameters which had been identified, in order to achieve the formation of a stable emulsion in a reproducible manner. In many situations, delays in the formation of the emulsion and a certain degree of instability of the emulsion were noted; such delays and/or instability resulted in a drop in the chromium and iron yields. In some cases even, no emulsion at all was formed.
A way of improving the reproducibility of initiation of the formation of the gas/molten pig iron emulsion, and also increasing the stability of the emulsion, once formed was therefore sought.